The field of the invention relates to speed control systems for motor vehicles.
Speed control systems are known for electrically controlling the engine throttle by a servo device such as a dc motor or vacuum operated diaphragm. In such devices a conventional feedback control system is utilized to control the servo in response to an error signal derived by subtracting vehicle speed from a desired speed.
Typical speed control systems also include a resume mode wherein speed control operation is temporarily disabled in response to operator braking. Upon operator actuation of a resume switch, speed control operation is reinitiated utilizing the previous desired or reference speed. Feedback control operates the servo in response to the error signal derived from the Previous reference speed and vehicle speed to accelerate the vehicle back to the previous reference speed. A problem with such systems is that the magnitude of the error signal may result in full throttle operation abruptly accelerating the vehicle and also overshooting the reference speed.
U.S. Pat. No. 4,337,511 issued to Schneider et al addresses the above problem by incrementing or ramping the desired speed to the previous reference speed upon actuation of the resume mode. Two ramps are utilized, the second ramp being more gradual than the first ramp in an attempt to gradually approach the previous reference speed.
The inventors herein have recognized numerous disadvantages of prior approaches in resume control or other modes of control in which the vehicle is accelerated by the speed control system. For example, the prior approach ramping of desired speed in a preprogrammed manner to approach the previous reference speed appears to be programmed for a specific vehicle operating under known road conditions, typically a flat road. Accordingly, performance may be degraded when road conditions vary such as when operating on a gradient. If for example the grade is uphill, the reference ramp may result in too slow a convergence to the resume speed. On the other hand, when operating downhill considerable speed overshoot may result. Further, the operator may deliberately accelerate to more rapidly converge on the resume speed resulting in speed overshoot. In addition, the controlled vehicles will vary considerably in performance characteristics such as horsepower and drivetrain gear. The control ramps must therefore be preprogrammed for each type of vehicle. Despite such effort, however, the vehicles response will vary considerably when loaded such as when hauling a trailer or carrying a heavy load. Accordingly, convergence to a resume speed may be unsatisfactory in prior approaches under a wide combination of road conditions, vehicle characteristics, and operator action.